Video technology involves electronically recording, storing, transmitting, and playing or displaying a sequence of still images called frames, in a time order sequence, in order to give the appearance of scenes in motion.
A digital video frame is represented using a grid of square, rectangular, circular, or non-uniform pixels. Each pixel is a sample of an original image, and a pixel (also known as a pel or picture element) is a single point in a raster image, the smallest addressable screen element, or the smallest unit of an image that is possible to be represented and controlled. Each pixel has a unique address, represented as coordinates on a two-dimensional grid. The intensity of each pixel may be variable.
The number of pixels or the density of pixels in a frame is called the resolution. In general, the larger the density of pixels (the higher the resolution), the sharper the image, and the more accurate the representation of the original image. The resolution may be expressed as a number of pixels per unit distance or area, such as the number of pixels per square inch.
Over the years, the technology of video display has progressed from television sets that receive broadcast signals, to cable television, to satellite television, to on-demanded video streamed over a network and displayed on a client, such as a television set, computer display, or telephone. Previously, video display was restricted to passively displaying the content that was broadcast from a sender to the television. But, today, on-demand video allows the viewer to selectively pause, rewind, or fast-forward the video. The client often pre-fetches (and the server sends) portions of the video, prior to the time that the video is played, in order to provide a sufficient amount of content available for immediate viewing. The client often does not pre-fetch, and the server does not send, the entire video prior to the start of viewing because of client memory constraints, network bandwidth constraints, and/or the desire of the content owner not to provide a complete copy of the video.